Bacteria in the gut may regulate weight gain and serum cholesterol levels by producing proteins that modify bile acids in the intestine, says a new study with implications for probiotics.

Researchers from University College Cork in Ireland report that bile salt hydrolase (BSH) is commonly made by gut bacteria and functions to change the chemical properties of bile acids in the gut. Data has indicated that specifically increasing levels of this protein reduces serum cholesterol levels and weight gain in mice.

The findings are published in the Proceedings of the National Academy of Sciences USA (PNAS).

“The findings may be used as a basis for the future selection of probiotics or dietary interventions which target this mechanism to regulate weight gain or high cholesterol,” said Dr Susan Joyce, lead author on the paper. “We now have the potential for matching probiotic strains with specific end-user needs. Work is underway to determine how this system operates in humans.”

According to the FAO/WHO, probiotics are defined as "live microorganisms which when administered in adequate amounts confer a health benefit on the host".

Gut health and obesity

The study adds to emerging body of science supporting the effects of gut microflora on metabolic factors and obesity.

In 2006, Jeffrey Gordon and his group at Washington University in St. Louis reported in Nature (Vol. 444, pp. 1022-1023
, 1027-1031
) that microbial populations in the gut are different between obese and lean people, and that when the obese people lost weight their microflora reverted back to that observed in a lean person, suggesting that obesity may have a microbial component.

Dr Gordon and his group recently pushed back the scientific boundaries even further in this area. In an ‘elegant’ study, the St Louis-based scientists reported that probiotics in a yogurt did not colonize the gut microflora when studied in identical twins, but additional study in mice revealed that ingestion of probiotic bacteria produced a change in many metabolic pathways, particularly those related to carbohydrate metabolism (Science Translational Medicine, Vol. 3, 106ra106).

Study details

One takes probiotics, the other probably should, too...

The new study from Ireland sought to build on the knowledge that the microbiota is linked to fat tissue accumulation and weight gain by elucidating a potential mechanism of action.

Using lab mice, Dr Joyce and her co-workers found that bile salt hydrolase, which is produced by gut bacteria, can significantly influence lipid metabolism, weight gain, and cholesterol levels. This microbial BSH activity was found to influence signalling pathways in the host that are linked to lipid metabolism, circadian rhythm, and epithelial cell function.

“High-level expression of BSH in conventionally raised mice resulted in a significant reduction in host weight gain, plasma cholesterol, and liver triglycerides, demonstrating the overall impact of elevated BSH activity on host physiology,” they wrote. “We demonstrate that bacterial BSH activity significantly impacts the systemic metabolic processes and adiposity in the host and represents a key mechanistic target for the control of obesity and hypercholesterolemia.”

This was followed by a nine week study, published in the European Journal of Clinical Nutrition
(2012, Vol. 66, pp. 1234-1241), which found that L. reuteri NCIMB 30242 capsules could reduce LDL and total cholesterol by 11.6% and 9.1%, respectively, in people with elevated cholesterol levels.

The company has not made any weight management claims, only relating to cholesterol levels.